1. Field of the Invention
This invention relates to magnetic disc members for use in magnetic disc devices which are a kind of a magnetic recording device; and it also relates to a process for manufacturing the same.
2. Description of the Prior Art
In recent years, a magnetic disc device in a file memory has risen in importance and its recording density has been remarkably improved year by year. Until now, as for a magnetic recording medium, there has been broadly used a so-called coating-type disc which was obtained by coating a mixture of iron oxide magnetic powders and an organic resin binder on a base plate and thereafter polishing the coating. In the coating-type disc, a lubricant can be incorpolated in the kneaded layer of the magnetic powder and the organic resin binder. In addition, a magnetic layer is not less than 0.5 .mu.m in thickness and the surface roughness R.sub.max is relatively large and is about 0.05 .mu.m which correspond to about 10% of a thickness of the magnetic layer (R.sub.a is about 0.0 .mu.m corresponding to only a fraction of R.sub.max). Thus, a lubricating film having enough thickness can be applied with no adsorption (stick) to a magnetic head and duarability to a contact and slide with the head is good. However, in order to attain the higher recording density, it is necessary to thin the magnetic recording layer. The coating-type disc is disadvantageous in that respect.
Thus, a thin film magnetic disc has began to be used as the high density recording medium. The magnetic recording layer is composed of a magnetic thin film formed by a plating process, a sputtering process, a depositing process or the like.
As for the magnetic disc member in the thin film magnetic disc, an aluminum alloy member or the like has been used heretofore. Ni alloy plating film is formed on a base plate which is followed by polishing the resulting surface. Generally, the member which is obtained by forming the Ni alloy plating film has a thickness of a few .mu.m to several tens .mu.m on the aluminum alloy base plate. Thereafter, a mirror-polishing is given to the surface of the film to a surface roughness R.sub.a of not more than about 0.04 .mu.m according to an abrading process such as a lapping or polishing process or the like (The Research Institute of Electrical Communication, practical Reports on Research 26-2, p.471, 1977).
However, in case of forming the magnetic thin film on the above member, there is a problem because a very small amount of a lublicant can only be retained at the surface of a magnetic disc and thus a CSS (Contact Start Stop) durability of the magnetic head is bad, since the magnetic thin film having a thickness of only a fraction of that of the magnetic recording layer in the coating-type disc is generally smooth and uniform. Thus there is no retention place for the lubricant at the surface of the magnetic disc. In addition, in the case that a liquid lubricant was coated on the mirror-polished surface of the magnetic disc, there was a defect because adsorption was caused between the lubricant coating and a magnetic head slide having a smooth surface and thus damage was easily caused when the disc started.
Therefore, there is a process wherein fine textures in the form of concentric circles are formed by contacting a tape including abrasive grains while rotating the Ni alloy plated member having the mirror-polished surface as mentioned above and a process for increasing the surface roughness wherein the Ni alloy plated member is once mirror-polished with fine abrasive grains and thereafter is polished with coarse abrasive grains.
However, in the process wherein the coarse surface is formed by means of the above machining techniques, heavy polished textures may cause signal errors because there are relatively large grains in the abrasive grains, foreign matters included therein when abraded or the like. It is difficult to obtain roughened surface which is uniform and suitable over the entire area of the disc. There is a problem that this non-uniformly polished textures have an influence upon the characteristics of the magnetic layer and thus that deterioration of the recording and reproducing characteristics such as resolution, S/N and the like. The servosignal quality deteriorates. Furthermore, in the machining techniques, it is impossible to control strictly the surface roughness. It is difficult to increase the density of the abrasive texture, since the obtained surface roughness is practically determined by the size of the abrasive graine used. Therefore, in order to prevent adsorption to the head and to give the lubricating film having a sufficient thickness, it is necessary to make the surface roughness fairly large. Thus there is a problem that deterioration of electromagnetic conversion characteristics is caused. In addition, the above precise machining has difficulty in a package mass production and has a problem in productivity.